In the preparation of printed circuit boards, it may be necessary to drill many openings which must be precisely located and have accurate dimensions. Tests have shown that the spindle must rotate at high rpm if good hole quality is to be achieved. The higher the rotational speeds, the better accuracy of the drilling and the longer the life of the drills. Conventional spindles are not capable of achieving optimum rotational velocity. A drilling spindle with conventional ball bearings is severely limited in the rotational speeds it can achieve. Spindles have been devised with air bearings, which allow higher speeds. Nevertheless, speeds beyond their capability--generally around 100,000 rpm--are desirable.
An additional serious difficulty has been encountered with spindles having air bearings. These spindles are delicate and generally cannot withstand forces other than those incurred during an ideal drilling operation. Other loads, such as impacts which may result from an operator error, cause metal-to-metal contact at the air bearings and instant destruction of the spindle. Such occurrences are widespread.
Heat is another enemy of hole quality and also of drill life. With prior art spindles, the drills not only become heated from friction while drilling the holes, but also receive heat from the spindle. Although the stator of the spindle drive motor may be cooled by an external water jacket, there has been no provision to cool the rotor, which increases in temperature as the spindle is operated. Heat from the rotor then becomes transmitted through the collet to the drill. This heat also results in dimensional growth of the rotor and results in electrical losses.
Spindles having tapered rubber bearings using water as a lubricant are capable of relatively high rotational velocities and are more durable than those with air bearings. Such bearings are disclosed in U.S. Pat. Nos. 3,929,393 and 4,229,139. However, these bearings as constructed and arranged in the past have not permitted a spindle to rotate as fast as desired. At very high rotational speeds, it is not possible to maintain a water film between the journal and the bearing surface in prior water bearing designs As water is introduced through the bearing, centrifugal force generated by the high speed rotation drives the water out of the bearing.
In U S. Pat. No. 3,929,393 one of the bearings is movable and is spring loaded to compensate for dimensional changes brought about by water absorption, wear and temperature changes. However, this construction will not function in a spindle that rotates at very high speeds. The end load on the bearing will be excessive at start-up and during low speed operation if the end load is to be adequate for high speed rotation. Too much end load will prevent the creation of a lubricating water film and can result in friction of a magnitude that will prevent the spindle motor from starting. A smaller end load, on the other hand, results in excessive vibration as speeds increase and will prevent high rotational velocities from being achieved.
Spindles for printed circuit board drilling machines are provided with automatically operated collets which function with tool changers to enable different sizes of drills to be operated during a drilling program. An improved collet for holding the drill bit is disclosed in U.S. Pat. No. 4,762,447 which employs self-holding tapers to retain the collet in the closed position gripping the drill shank. This avoids the need for springs or some other means for maintaining the collet in the closed position as the drilling operation proceeds. This collet suffers from a problem, however, in the event that the drill bit breaks off flush with the bottom end of the collet. There is then no way of gripping the stub end of the drill to pull it from the collet. Also, the collet cannot then be removed from the shaft of the rotor without a major disassembly. Consequently, dismantling of the spindle becomes necessary for removing a broken drill bit under those circumstances